Water is transported in a variety of metal pipes which include copper, iron and steel pipes. These pipes corrode when exposed to water, even pure water. Most water, however, even drinking water, has salt impurities that promote corrosion. As a result corrosion control agents often are added to the water being conveyed in metal pipes. Zinc phosphate, which is an orthophosphate, is such a known corrosion control agent. It is known that when zinc orthophosphate is added to water being conveyed in water pipes at rates that will result in from about 0.1 to about 10 ppm of zinc phosphate in the water in-the pipes, the zinc orthophosphate will deposit a corrosion inhibiting film on the interior surface of the pipes. Current sources of zinc phosphate, however, often are salts such as zinc chloride and zinc sulfate. These salts are sources for contaminant chloride and sulfate anions in solutions of zinc orthophosphate. Even though the contaminant are ions at low concentrations, they are not desirable. Ironically, chloride ions are known to cause corrosion of metals. Sulfate ions often precipitate from solutions of zinc orthophosphate as insoluble inorganic sulfates which is not desirable because such precipitates are solids which eventually block pumps and pipes and threaten the functionality of storage tanks.
Additionally it is desirable to completely minimize sources of contaminants from any water additive to inhibit and control corrosion in metal water pipes because metal pipes often convey drinking water. Hence, care should be taken that the corrosion control additive will eliminate or at least minimize contaminants such as cadmium, copper, chromium, cobalt and mercury.
It is an object of the invention to provide a method for providing a corrosion inhibiting solution which is substantially free of contaminants such as chloride and sulfate ions.
It is another object of the invention to provide a method of making a solution of zinc orthophosphate which is substantially contaminate free.
It is still another object of the invention to provide a method of inhibiting the corrosion of metal pipes using a contaminant free source of zinc orthophosphate.
These and other objections of the invention will become apparent with reference to the specification herein.
The invention is directed to a method for providing a corrosion inhibiting aqueous solution which includes zinc orthophosphate where the zinc orthophosphate is solubilized and coats the internal surfaces of water pipes and inhibits the corrosion of such surfaces. The invention is particularly suited for use in connection with pipes which transport drinking water. Further, the invention also involves a method of making a substantially contaminant-free, concentrated aqueous solution of zinc orthophosphate from zinc metal, or zinc oxide, and aqueous phosphoric acid, and in an important aspect, from zinc metal or zinc metal oxide, aqueous phosphoric acid and an oxidizing agent as needed.
In the aspect of the reaction of zinc metal with phosphoric acid, the oxidizing agent is used to take the zinc metal (Zn0) to zinc 2+(Zn2+). The oxidizing agent is not necessary to the reaction of zinc metal oxide with phosphoric acid. The oxidizing agent is in an amount which is effective for promoting the reaction of the zinc metal and aqueous phosphoric acid. Generally this is a stoichiometric amount of oxidant, e.g., one equivalent of oxidant for one equivalent of zinc metal (2 moles of oxidant to get one mole of Zn2+). In an important aspect, the oxidizing agent comprises less than about 5 weight percent of the solution of zinc metal and phosphoric acid which is reacted to form the zinc orthophosphate. During the reaction, the oxidant is consumed and oxidant should not be used in any excess that would create more than about 3 weight percent total contaminants in the zinc orthophosphate solution, and preferably less than 1 weight percent contaminants.
The method for supplying a corrosion inhibiting solution includes mixing zinc ion source of zinc metal or zinc metal oxide with aqueous phosphoric acid. The zinc ion source/phosphoric acid mixture has at least less than about 3 weight percent impurities; and in an important aspect, less than about 0.1 to 3 weight percent impurities. The aqueous phosphoric acid is in an amount which is effective for providing a solution having a pH of less than about 2 to provide a corrosion inhibiting concentrate solution. The zinc ion source is in an amount which is effective for providing the concentrate with at least about 0.05 weight percent zinc ions; and in an important aspect, from about 0.1 to about 25 weight percent zinc ions. Generally, the phosphoric acid is in an amount which is effective for providing a phosphate ion concentration of at least about 10 weight percent in the concentrate solution, and in an important aspect, from about 10 to about 70 weight percent phosphate ions. The zinc ion source and the phosphoric acid each should be of a purity such that they should each be substantially free of impurities, that is less than about 1 weight percent impurities, especially substantially free of chloride ion and sulfate ion. To inhibit corrosion, the corrosion inhibiting concentrate solution is mixed into water which is being directed into the metal pipes at a rate which will be effective for providing a deposit of zinc and phosphate film onto the interior surface of the pipes. In one aspect of the invention, the zinc orthophosphate should be in an amount which is effective for providing from about 0.1 to about 10 (ppm) zinc orthophosphate in the water being transported in the metal pipes.